1. Field of the Invention
The present invention relates to an electronic device, such as a display device or an integrated circuit device, having a thin film transistor. Also, the present invention relates to a method of manufacturing the electronic device including the step of annealing a thin film semiconductor layer of the thin film transistor by carrying out laser beam scanning.
2. Description of the Related Art
A display device such as a liquid crystal display device or an organic EL (electroluminescence) device is known as an electronic device having a Thin Film Transistor (TFT).
The thin film transistor (hereinafter referred to as “the TFT”) is used as a switching element which applies a voltage corresponding to an image signal to a liquid crystal layer in the liquid crystal display device.
In addition, the TFT is also used as a drive transistor which controls an amount of current caused to flow through a self light emitting element provided every pixel in the organic EL display device. In each of the pixels of the organic EL display device, in addition to the drive transistor, a switching element is necessary for the cutoff control of a current path, sampling for an image signal, or the like. The TFT is also used as the switching element.
The TFT has a thin film semiconductor layer made of polysilicon, amorphous silicon or the like within a lamination structure obtained by laminating thin films on a panel substrate of a display device by utilizing the same technique as that for a semiconductor integrated circuit. A source/drain region is formed in the thin film semiconductor layer, and a gate electrode is disposed so as to be close to either an upper surface or a lower surface of the thin film semiconductor layer through a gate insulating film.
In general, in a low-temperature polysilicon TFT, an Exima Laser Anneal (ELA) method is utilized in an anneal process for crystallizing amorphous silicon (a formation film of the thin film semiconductor layer) to cause amorphous silicon to turn into polycrystalline silicon (polysilicon).
However, although the low-temperature polysilicon TFT formed by utilizing the ELA method has a large mobility, threshold voltages or the mobilities of the low-temperature polysilicon TFTs largely disperse. The threshold voltages and the mobilities of the TFTs disperse, which results in that an operation for the pixel drive becomes unstable in the liquid crystal display device or organic EL display device described above.
More specifically, in the laser beam radiation utilizing the ELA method, line-like scanning is carried out in such a way that the laser beam is intermittently radiated to a region (shot region), having a predetermined shape, radiated with a laser beam in accordance with pulse drive while the region radiated with the laser beam is partially shifted in one direction. Also, the next line-like scanning is similarly carried out in such a way that a center of the shot region is shifted in a direction perpendicularly intersecting the one scanning direction so that the shot region partially overlaps the former shot region in the direction as well perpendicularly intersecting the one scanning direction. The ELA method is carried out by repetitively carrying out such line-like scanning using the laser beam.
At this time, for example, the uniformity of the TFT characteristics within the scanning line becomes superior to that of the TFT characteristics in the direction perpendicularly intersecting that scanning direction. To put it the other way around, the threshold voltages and mobilities of the TFTs are easy to disperse in the direction perpendicularly intersecting the scanning line, which causes periodic streak-like luminance nonuniformity on the display screen.
In particular, in the drive transistor of the organic EL display device, an amount of drive current for a light emitting element is adjusted in accordance with a level of a signal inputted to the pixel. As a result, the dispersion of the threshold voltages and mobilities of the TFTs directly appear in the form of a change in emission luminance of the pixel. In addition, although the sampling transistor is the switching transistor, the sampling time depends on the threshold voltage, and when the mobility differs, an amount of signal charges fetched in differs even for the same sampling time. Therefore, the emission luminance changes in any way due to the dispersion as well of the characteristics of the sampling transistors.
In order to prevent the display nonuniformity in the organic EL display due to the ELA scanning, various kinds of proposals are made. These proposals, for example, are described in Japanese Patent Laid-Open Nos. 2003-91245 and 2002-175029 (hereinafter referred to as Patent Document 1 and Patent Document 2).
In the technique described in Patent Document 1, a plurality of TFTs (drive transistors) are provided so as to be connected in parallel with light emitting elements, respectively. At this time, a direction (channel length direction) along which a channel current is caused to flow is made to differ among the plurality of TFTs. Also, the ELA laser beam radiation in one direction, or the ELA laser radiation in plural directions is carried out for the amorphous semiconductor film a crystalline property of which differs depending on the channel length directions, thereby making the periodical luminance nonuniformity due to the ELA scanning described above less noticeable on the display screen.
In Patent Document 2, a compensating transistor which has a channel conductivity type opposite to one conductivity type of a drive transistor and which is diode-connected is provided in series with the drive transistor connected to a light emitting element. When the characteristic dispersion is caused in the drive transistor due to the laser beam line-like scanning in the phase of the ELA, the opposite characteristic dispersion is caused in the compensating transistor of the opposite conductivity type so as to absorb the change in drive current due to the characteristic dispersion described above. For this reason, the periodic luminance nonuniformity due to the ELA scanning described above can be made less noticeable on the display screen.